On Wed, Mar 9, 2022 at 10:48 PM Christoph Hellwig [off-list ref] wrote:

On Tue, Mar 08, 2022 at 10:42:09PM -0800, Song Liu wrote:

quoted

RAID arrays check/repair operations benefit a lot from merging requests.
If we only check the previous entry for merge attempt, many merge will be
missed. As a result, significant regression is observed for RAID check
and repair.

Fix this by checking more than just the previous entry when
plug->multiple_queues == true.

But this also means really significant CPU overhead for all other
workloads.

Would the following check help with these workloads?

 if (!plug->multiple_queues)
              break;

quoted

This improves the check/repair speed of a 20-HDD raid6 from 19 MB/s to
103 MB/s.

What driver uses multiple queues for HDDs?

Can you explain the workload submitted by a md a bit better?  I wonder
if we can easily do the right thing straight in the md driver.

It is the md sync_thread doing check and repair. Basically, the md
thread reads all
the disks and computes parity from data.

Maybe we should add a new flag to struct blk_plug for this special case?

Song